Where Do Flies Go in Winter? Cluster Flies, House Flies and Gnats
Most flies do not go anywhere in winter: outdoor adults die off, and the species survives as maggots and pupae in manure, compost and other warm rotting material until spring. House flies work this way across the northern US, and keep breeding all year in warm climates and heated barns. Cluster flies, face flies and the black blow fly are different: they spend winter as dormant adults in attics and wall voids, which is why sluggish flies appear at windows on sunny winter days. Small flies indoors in winter, such as fungus gnats and fruit flies, are usually breeding inside.
The answer depends on which fly you mean: some sit it out as larvae, some hide as adults in the structure of your house, and a few never stop breeding.
Where each fly spends the winter
| Fly | Winter stage | Where it spends winter | Seen indoors in winter? |
|---|---|---|---|
| House fly | Larvae and pupae; some adults | Under manure piles and organic debris outdoors; adults in heated buildings | Occasionally, one or two from a warm space |
| Cluster flies | Dormant adults | Wall voids, attics, upper storeys; also under bark and in rock crevices | Yes, on warm sunny days |
| Face fly | Hibernating adults | Crawl spaces, attics and false ceilings, often on south and west walls | Yes, near windows, mostly near cattle pasture |
| Black blow fly | Dormant adults (diapause) | Buildings, like cluster flies | Sometimes |
| Blue bottle flies | Adults, not fully dormant | Buildings; may keep breeding when warm enough and food is available | Sometimes |
| Little house fly | Adults seek cover | Buildings or dense vegetation as temperatures fall | Occasionally |
| Fungus gnats | All stages, breeding | Damp houseplant soil indoors | Yes, often most noticeable in late autumn and winter |
| Fruit flies | All stages, breeding | Fermenting produce, drains, recycling indoors | Yes, year round where food is left out |
The pattern to take from the table: a big, slow, grey fly at a January window has usually been in the house since autumn, while a cloud of tiny flies in January is breeding somewhere inside now.
Do flies die in winter?
Individual adult flies outdoors mostly do, but the species does not. Flies go through egg, larva, pupa and adult, and in cold regions the immature stages carry the population through. University of New Hampshire Extension says that in New Hampshire house flies overwinter outdoors as pupae in manure piles and organic debris, and that some adults survive the winter in heated buildings. University of Nebraska Extension gives the same picture for the Plains: the house fly overwinters as larvae or pupae under manure piles or in other breeding areas.
Heat changes everything. Nebraska notes that in milk barns, hog houses or confined feeding units that are closed and warm, house flies may continue to develop through the winter.
Do flies hibernate?
Some do, and they are the ones that cause winter fly complaints. Colorado State University Extension describes cluster flies entering a dormant state called diapause from early autumn: they stop reproducing, slow their metabolism and stay inactive for months, stirring a little during warm sunny spells. Shortening day length in late summer is the trigger that switches adults from breeding to looking for shelter.
The black blow fly also goes into full winter dormancy, according to Colorado State, while blue bottle flies do not become fully dormant and may reproduce year round when temperatures and food allow. UC IPM adds the face fly: in late autumn large numbers may enter buildings to hibernate through winter, becoming active on warm days.
Where do flies go in winter in the house?
Into the structure, not the living space. Colorado State reports that cluster flies start resting on sun-warmed south and west walls as early as mid-August, enter through gaps around windows, soffits, vents and siding, and settle in wall voids and attics, concentrating on upper storeys. “Attic fly” is another name for them. UC IPM gives the same search advice for face flies: start on the southern and western sides of the building, because those walls get the most winter sun, then check crawl spaces, attics and false ceilings.
Penn State Extension is explicit that cluster flies are not reproducing inside the structure; they crawl out of voids on warm days in a confused attempt to get back outside. Colorado State adds that most do get out, in early spring.
The full checklist for telling cluster flies from a dead animal or a breeding source is in our guide to flies in the house all of a sudden, and the wider group of autumn invaders, from lady beetles to stink bugs, is covered in bugs in the house in winter.
Why are there flies in my house in winter?
Match what you see to the right row before you buy anything.
| What you see | Most likely fly | What it means | First step |
|---|---|---|---|
| Slow grey flies at upper windows on sunny days | Cluster flies | Overwintering adults in walls or attic; not breeding | Vacuum them; seal gaps next August |
| Similar flies, rural home near cattle | Face flies | Hibernating in the structure | Check south and west walls and attic |
| Tiny black flies around houseplants and windows | Fungus gnats | Breeding in wet potting soil | Let the top 1 to 2 inches of soil dry |
| Tiny tan, red-eyed flies in the kitchen | Fruit flies | Breeding in fermenting food or residue | Find and remove the source |
| Shiny blue or green flies, several at once | Blow flies | Possibly a dead animal nearby, or wintering adults | See our sudden flies guide |
| One ordinary house fly | House fly | A stray from a warm space or a door opening | Swat it; no treatment needed |
Fungus gnats deserve a special word, because winter is their season indoors. Colorado State explains why nuisance problems tend to peak in late autumn and winter: plants brought in from outside carry gnats with them, people are indoors more, and above all shorter days and cooler temperatures slow plant growth and water use, so soil watered on a summer schedule stays wet. At room temperatures of 65 to 75°F, the egg to adult cycle takes three to four weeks, with continuous breeding on indoor plants. NC State Extension notes they also turn up in little-used floor drains, basement bathrooms and other damp spots. Our guides to what attracts gnats and small flying bugs in the house cover identification, and fruit flies, drain flies and gnats separates the three.
Fruit flies are a year-round problem wherever produce is left out. University of Kentucky entomologists describe them as about 1/8 inch long, able to lay about 500 eggs, and able to go from egg to adult in about a week.
Which flies are still around in winter: what the records show
We counted when people in the US logged research-grade records of five household flies on iNaturalist, month by month. Comparing flies within the same month is fair, because every fly faces the same number of people out looking that month.
In January, people recorded 170 cluster flies, 252 blow flies, 81 fruit flies and 33 house flies. In February the cluster fly count rose to 320 while house flies fell to 29, and by March cluster flies reached 1,013, as dormant adults woke and headed out. House flies stayed low until spring: 222 records in August against 33 in January. That fits the biology: house flies wait out winter as pupae, while cluster flies are adults that can be seen whenever a warm day stirs them.
A second way to read the same data is the share of each fly’s yearly records that fall in December, January and February. Fruit flies lead, at 13.1 per cent, followed by cluster flies at 10.1 and drain flies at 9.5; house flies sit at 7.2 and blow flies at 4.0. Fruit flies and drain flies breed indoors, so part of their winter share reflects people noticing them inside when little else is about, not more breeding in winter. These are records of where and when people photographed and identified flies, not infestation counts.
Where do flies go in winter in Texas?
In much of Texas, house flies do not need to go anywhere. Nebraska Extension notes that in warmer climates the house fly breeds throughout the year, so in a mild Texas winter flies slow down rather than vanish, picking up again on warm spells. Texas A&M AgriLife entomologist Mike Merchant points out that house flies rarely breed inside structures; they come in through open doors and unscreened windows for food or shelter, so winter house flies in a Texas home usually point to a breeding source outside, such as garbage or pet waste.
Cluster flies, the classic northern winter nuisance, are scarce in the South.
In our analysis of iNaturalist records by state, cluster flies made up 3.5 of every 10,000 insect records in Pennsylvania, from 267 records, against 0.2 in Texas, from 57 records; Florida had none in our pull. A Texan with winter flies is more likely to have a breeding source than a wall full of hibernating cluster flies. Our seasonal guides to common house bugs in Texas and Pennsylvania show how different the two winters are.
Why do flies disappear when it’s cold?
Because fly development runs on temperature. UC IPM notes that under optimal summer conditions a house fly can go from egg to adult in as little as 7 days, and University of New Hampshire Extension says the adult emerges from the pupal case in three days to four weeks depending on temperature and humidity. As autumn cools, each generation takes longer, adults die and are not replaced as quickly, and the population drops to a scatter of pupae buried in warm material.
For the hibernating flies, cold is not even the main cue. Colorado State says cluster flies shift their behaviour as day length shortens in late summer, which is why they start gathering on sunny walls in August and September, while the weather is still warm.
Where do flies go at night?
House flies rest. Nebraska Extension says they rest on sunny surfaces in the daytime and tend to come inside buildings or under eaves to roost at night, and that small dark fly specks mark where they roost. UC IPM adds that house flies prefer light-coloured resting surfaces. If you see a fly sitting still on a ceiling or high on a wall after dark, it is roosting, not stalking you.
Where do flies come from in spring?
From the overwintered stages. Pupae that sat out winter in manure and debris finish developing as things warm, and each new female can lay several batches of about 100 to 150 eggs in manure, garbage, lawn clippings or rotting fruit, UC IPM says, which is why numbers climb fast after the first warm spell. Stage timings are in how long flies live. Cluster flies return to lawns, where their larvae develop inside earthworms.
What gets rid of flies permanently?
Nothing removes flies for good, but the right step for each fly comes close.
- House flies and blow flies: UC IPM says removing the material larvae develop in is the preferred method, and insecticides are no substitute for sanitation. Lidded bins, prompt removal of pet waste and screens are usually enough at home.
- Cluster flies: sealing is the only lasting fix. Colorado State says flies begin moving into buildings in late August, most in September, so gaps should be sealed by mid-August and no later than Labor Day, focusing on sunny south and west sides and upper areas.
- Do not treat the voids yourself. Penn State advises against using insecticides inside the structure once flies are in the walls, because dead flies in voids can feed carpet beetles that then attack woollens and stored goods, and foggers will not stop flies emerging from walls. Void dusting is specialist work for a licensed professional, and so is sealing high eaves and soffits you cannot safely reach.
- Lawn sprays do not prevent cluster flies. Colorado State states plainly that insecticides applied to lawns have never been shown to kill developing cluster flies.
- Fungus gnats: let the top 1 to 2 inches of potting mix dry between waterings and repot plants whose mix has broken down, Colorado State advises.
If dead flies are piling up in an attic, vacuuming them out also removes food for carpet beetles; our guide to carpet beetle treatment costs explains what happens if that step is skipped.
Frequently asked questions
What smell do flies absolutely hate?
No smell has been shown to keep flies out of a home. Extension guidance such as UC IPM’s fly Pest Note relies on screens, sealed gaps, lidded bins and removing breeding material rather than scents. Cluster flies are a further problem for scent-based ideas: Colorado State notes they are not drawn to most fly traps with food or chemical lures, though light traps in attics can catch them. Sealing entry points before late August works better than any odour.
What is the lifespan of flies?
An adult house fly usually lives 15 to 25 days and can reach about two months, but only two to three days without food, as our guide to how long flies live sets out from university sources. Temperature drives the whole cycle, so summer generations turn over in a week or two. Overwintering cluster flies are the long-lived exception: adults that shelter in buildings in autumn survive for months in dormancy and leave in spring.
Do flies hibernate in the walls?
Some do. Cluster flies, face flies and the black blow fly spend winter as dormant adults in wall voids, attics and crawl spaces, according to Colorado State and UC IPM. They are not breeding there; Penn State says they become active on warm days and crawl into rooms trying to get back outside. House flies do not hibernate in walls; they overwinter as larvae and pupae in manure and debris outdoors.
What is the song Where Do Flies Go in the Winter Time?
It is a comic British music hall song, “Where Do Flies Go in the Wintertime?”, credited to Sam Mayo and Frank Leo. The IMSLP sheet music library lists it as first published in 1917, with a London edition from the Herman Darewski Music Publishing Company in 1919. The question in the title has a real answer: most go nowhere, waiting out winter as pupae, while some hide as adults inside buildings.
Sources
- University of New Hampshire Cooperative Extension, House Fly fact sheet.
- University of Nebraska Extension G958, House Fly Control Guide, John B. Campbell, revised 2006.
- University of California Statewide IPM Program, Pest Notes: Flies (publication 7457).
- Colorado State University Extension, Cluster Flies and other “Winter Flies”.
- Colorado State University Extension, Fungus Gnats as Houseplant and Indoor Pests.
- Penn State Extension, Cluster Flies.
- Iowa State University Extension and Outreach, Yard and Garden: Blow and Flesh Flies.
- University of Kentucky Entomology, Entfact 621: Fruit Flies.
- N.C. Cooperative Extension, Lee County Center, Fungus Gnats in February?, 2021.
- Texas A&M AgriLife Extension, Indoor Flies and Their Control (ENTO-050), Michael Merchant, and Texas IPM Program release, 2016.
- IMSLP Petrucci Music Library, Where Do Flies Go in the Wintertime? (Mayo, Sam).
- The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.
